1. Field of the Invention
This invention relates to a process for the desulphurization of carbonaceous materials, and more particularly to a process for the desulphurization of petroleum coke having a sulphur content of over about 5% by weight, to provide a final coke product having a sulphur content of less than about 1% by weight. Desulphurization of the coke is accomplished by heating coke particles sequentially in a plurality of individual, spaced fluidized beds that are connected by inclined ducts, so that the petroleum coke particles pass from one fluidized bed to a succeeding bed through the inclined ducts.
2. Description of the Related Art
Heating of petroleum coke particles to desulphurize the particles can be effected in several ways, including the passing of an electric current through a fluidized bed of the particles. Because such particles can conduct electric current, the amount depending upon their electrical resistivity, such a heating technique is the most efficient way to directly heat the particles. Moreover, electrical heating can provide high particle temperatures of over 3000.degree. C., and even as high as the temperature at which graphite sublimates (approximately 3700.degree. C.).
The electrical resistivity of petroleum coke particles varies over a wide range, from thousands of ohm-cm. for undevolatilized petroleum coke having over about 5% sulphur content by weight, to about 1.times.10.sup.-3 ohm-cm. for graphite that is obtained from petroleum coke. Devolatilized petroleum coke that contains over about 5% sulphur by weight has an electrical resistivity of approximately 10 ohm-cm. Low sulphur petroleum coke having a sulphur content of about 0.3% by weight, which can be obtained by desulphurizing petroleum coke having an initial sulphur content over 5% by weight, has an electrical resistivity of approximately 0.10 ohm-cm. Hence, the electrical resistivity of devolatilized but undesulphurized petroleum coke is on the order of about one hundred times higher than the electrical resistivity of desulphurized petroleum coke having a sulphur content of about 0.3% by weight. Further, desulphurized petroleum coke is partially graphitized.
If a high sulphur petroleum coke is to be continuously desulphurized by means of electrical heating in a single fluidized bed, as is suggested in U.S. Pat. No. 4,160,813, which issued July 10, 1979, to Markel et al., and if the coke particles are to act as an electrical resistance, it is necessary to have an electrical power system capable of providing a wide range of voltages and/or currents. The higher sulphur content particles entering the single fluidized bed have a higher electrical resistance, and when they are added to the partially desulphurized particles in the bed they sharply increase the electrical resistance of the totality of petroleum coke particles in the fluidized bed. Moreover, the electrical power delivered to heat the particles must be sufficient to permit the particles to reach a desulphurization temperature greater than about 1700.degree. C. to obtain a sulphur content less than 0.5% by weight in the fluidized bed.
A disadvantage when using a single fluidized bed for the continuous desulphurization of petroleum coke is that the residence times of the particles can vary considerably, and the sulphur content of the particles at the output of the fluidized bed is therefore less uniform. Additionally, when using a single bed for the continuous treatment of high sulphur petroleum coke, the fluidized bed temperature must be high (greater than about 1800.degree. C.), because the entering particles reach the temperature of the fluidized bed in seconds (in some cases the entering particles are preheated up to 1000.degree. C.), and the resulting desulphurized petroleum coke is adversely affected in that the porosity of the particles increases and the mechanical strength of the particles decreases. Furthermore, even if several zones in a single vessel are separated by partitions, as is suggested in U.S. Pat. No. 2,983,673, which issued May 9, 1961, to Grove, and each separate zone is independently heated, because of heat transfer between the zones it is very difficult to maintain different temperatures in each zone.